Supported chromium oxide catalysts are commonly employed to prepare polyolefins having desirable characteristics. Various supports for chromium oxide catalysts have been disclosed in the art. The particular support used for the chromium oxide strongly affects the properties of the polymer being formed. Silica supports have primarily been used due to their ability to form highly active polymerization catalysts. However, silica supports do not provide for the production of ultra high molecular weight polymers when hexavalent chromium is formed during the catalyst activation, which often occurs. Aluminum phosphate supports are similar to silica supports in that they form highly active catalysts. However like the silica supports, they also do not have the ability to produce very high molecular weight polymers. Further, the polymers produced using the aluminum phosphate supports tend to contain relatively high amounts of long chain branching, which is not always a desirable property during processing of the polymer.
While the possibility of using alumina supports for chromium oxide catalysts has been described in the art, such supports typically are not used. Alumina supports desirably have relatively high surface areas and are very porous; however, chromium oxide catalysts supported by alumina are not sufficiently active to be considered commercially viable. The activity of such catalysts can be improved by adding fluoride to the alumina support. It is believed that the fluoride replaces surface hydroxide groups, which are believed to interfere with polymerization. Unfortunately, the addition of too much fluoride tends to sinter the alumina, resulting in the deactivation of the catalysts. A need therefore exists to develop a method for increasing the activity of chromium catalysts supported by alumina without being concerned that the alumina might be sintered.